Solid state lighting arrays are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state lamps have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state lamp may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region.
Solid state lighting panels are commonly used as backlights for small LCD display screens, such as LCD display screens used in portable electronic devices. In addition, there has been increased interest in the use of solid state lighting arrays for backlights of larger displays, such as LCD television displays.
For smaller LCD screens, backlight assemblies typically employ white LED lamps that include a blue-emitting LED coated with a wavelength conversion phosphor that converts some of the blue light emitted by the LED into yellow light. The resulting light, which is a combination of blue light and yellow light, may appear white to an observer. However, while light generated by such an arrangement may appear white, objects illuminated by such light may not appear to have a natural coloring, because of the limited spectrum of the light. For example, because the light may have little energy in the red portion of the visible spectrum, red colors in an object may not be illuminated well by such light. As a result, the object may appear to have an unnatural coloring when viewed under such a light source.
The color rendering index of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources to nearly 100 for incandescent sources. Light generated from a phosphor-based solid state light source may have a relatively low color rendering index.
For large-scale backlight and illumination applications, it is often desirable to provide a lighting source that generates a white light having a high color rendering index, so that objects and/or display screens illuminated by the lighting panel may appear more natural. Accordingly, such lighting sources may typically include an array of solid state lamps including red, green and blue light emitting devices. When red, green and blue light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources. There are many different hues of light that may be considered “white.” For example, some “white” light, such as light generated by sodium vapor lamps, may appear more yellowish, while other “white” light, such as light generated by some fluorescent lamps, may appear more bluish in color.
Solid state lamps, such as LED's, are current-controlled devices in the sense that the intensity of the light emitted from an LED is related to the amount of current driven through the LED. One common method for controlling the current driven through the solid state lamps to achieve desired intensity and color mixing is a Pulse Width Modulation (PWM) scheme. PWM schemes pulse the solid state lamps alternately to a full current “ON” state followed by a zero current “OFF” state. However, firing all of the solid state lamps simultaneously may result in undesirable electrical power system performance due, for example, to power factor spikes and/or electromagnetic interference due to the associated current rush. Sequentially firing the strings independently and/or in combinations without regard to relative string location within the display, however, can produce undesirable display characteristics.